You're right about "dedicated to functions they provide" -- and thank you for recognizing THAT. But there is also an even more stringent focus. As you say, many of us are either webmasters or even website promoters. And on those sites, or for those sites, we pursue our own interests. Others of us do not have websites, but merely contribute to other sites. But, whether or not I have my own website or contribute to other websites or offer services to other sites, what I do AT THIS SITE needs to serve the purposes of THIS site.
What I do at THIS site must add to the content of THIS site--obviously. Also obvious (but often forgotten!), is that I shouldn't interfere with anyone else who's also contributing to THIS site.
How do we know what would cause other ODP contributors trouble? Well, unfortunately, a lot of it is experience in doing things that accidentally caused trouble.
But some things would cause obvious harm. Suppose someone set up a forum to discuss, say, the ODP. And suppose someone else wanted to use that forum to get special services for his website. What would happen?
I don't have to guess, I know. But anybody could guess. Every spammer in the Nine Circles of the Inferno would swarm down on that forum, intent on using it to get privileged services for their spam. The creatures from the abyss, who are already suggesting 5000 spam suggestions a day, would simply convert to posting 5000 spam posts a day. Any other conversation would be drowned out by the pandemonium.
End of forum.
As you say, there must be a better way of website promotion. I can't tell what it would be -- I'm neither skilled, trained, or interested in that (which is why I contribute to other kinds of projects instead.)
You're welcome to try to create a better way. As you say, many ODP editors are also active elsewhere on the web: those who are interested in website promotion might participate in your project, if they think it's worthwhile. But whatever value that goal has (I won't express my opinion, it doesn't matter) and whoever wishes to help with that goal (whether or not they also contribute to the ODP) -- in any case, the ODP itself isn't likely to be involved. Because it was set up for something else.
It's like -- the rural mail carrier may occasionally run over a squirrel or rat. But that doesn't mean the Post Office is in the varmint extermination business! And telling them to change their routes to kill more rats, isn't likely to be persuasive.